Using the preinstalled Google services effectively turns your smartphone into spyware. In the standard Android that comes preinstalled on most smartphones, Google services run with system privileges and can collect all kinds of data including location data. If Signal exclusively supports this, it reinforces dependence on Google and discourages people from using versions of Android that don’t include Google’s spyware.
I'm not sure if the comment above you is correct, but if it is (meaning Signal would benefit from the anonymity of every client being the same/similar, like TOR), then this would go against that.
That being said, I've never heard this mentioned as a benefit of Signal, and I figured the main benefit was y'know robust and proven E2E encryption.
Then don't do that?
Install vanilla LineageOS or GrapheneOS with no GApps. Then install Signal from https://signal.org/android/apk/